In video system applications, a picture is displayed on a television or computer screen by scanning an electrical signal horizontally across the screen one line at a time. The amplitude of the signal at any one point on the line represents the brightness level at that point on the screen. When a horizontal line scan is completed, the scanning circuit is notified to retrace to the left edge of the screen and start scanning the next line provided by the electrical signal. Starting at the top of the screen, all the lines to be displayed are scanned by the scanning circuit in this manner. A frame contains all the elements of a picture. The frame contains the information of all the lines that make up the image or picture and all the associated synchronization (sync) signals that allow the scanning circuit to trace the lines from left to right and from top to bottom.
There may be two different types of picture scanning in a video system. For some television signals, the scanning may be interlaced, while for some computer signals the scanning may be progressive or non-interlaced. Interlaced scanning occurs when each frame is divided into two separate, but temporally coherent, sub-pictures or fields. The interlaced picture may be produced by first scanning the horizontal lines for the first field and then retracing to the top of the screen and then scanning the horizontal lines for the second field. The progressive or non-interlaced picture may be produced by scanning all of the horizontal lines of a frame in one pass from top to bottom.
The portion of the frame that represents a horizontal line scan may have an active video portion and a horizontal blanking portion. The active video portion may contain brightness and color information for the horizontal line while the horizontal blanking portion may contain a horizontal sync signal. The horizontal blanking portion is displayed as black and is not seen on the screen while the scanning circuit retraces to the starting point of the line on the screen. Similarly, a frame may have a portion of the video signal represent a vertical blanking portion. The vertical blanking portion may contain a vertical sync signal and is displayed as black and is not seen while the scanning circuit retraces from the bottom to the top of the screen in order to start a new field or frame.
The video signal displayed in the active portion of the horizontal line scan may contain red (R), green (G), and blue (B) video components to represent a point on the line. In some instances, it may be preferable to convert the R, G, and B video components to a different set of color coordinates in order to reduce the amount of color information in the video signal. Transformation to a different color coordinate with reduced color information may be possible because of the poor color acuity of the human vision system. In component analog video, for example, it may be preferable to represent a picture by its luma (Y) and its color difference components Pb and Pr than by its R, G, and B video components because of limitations in transmission bandwidth.
Component video signals, in certain configurations, may have three independent sync signals, where each one corresponds to a different color component. In most instances, a video decoder unit is adapted to ignore two of those independent signals and select one for synchronizing the horizontal and vertical blanking of all color components in the video signal. The video channel carrying the Y color component or the G color component may be used as reference for synchronization because most of the color information in a video signal is in one of these color components. However, the quality of the displayed picture suffers due to the misalignment that occurs as a result of inadvertence or other failure to incorporate the sync information contained in the two additional component video signals.
Further limitations and disadvantages of conventional and traditional approaches will become apparent to one of skill in the art, through comparison of such systems with some aspects of the present invention as set forth in the remainder of the present application with reference to the drawings.